1. Technical Field of the Invention
The embodiments of the invention relate to converter circuits and, more particularly, to utilizing a PFM circuit to control a DC-DC converter of an integrated circuit chip.
2. Description of Related Art
A variety of electronic equipment, especially portable or handheld electronic devices, are capable of being powered from a battery. Some of these portable and/or handheld devices include, but are not limited to, laptop or notebook computers, personal digital assistants (PDAs), compact disc (CD) players, digital video disc (DVD) players, MP3 (an audio standard under the Moving Picture Experts Group or MPEG) players, AM/FM (amplitude modulation/frequency modulation) radios, pagers, cellular telephones, etc. These devices typically contain one or more integrated circuit chips to perform the function(s) intended by the device. Many, if not most, of these devices may be powered by self-contained power sources (such as a battery), and many are powered by external power sources (wall outlet), or both.
In today's applications, certain interfaces may provide a power source through the interface itself. A bus specification, such as a Universal Serial Bus Revision 2.0 specification (USB 2.0) provides not only protocol for data transfer between two devices, but the USB 2.0 specification also provides a power link between a USB host and a USB device coupled onto the bus. For example, a peripheral device (such as a MP3 player), when coupled to a host (such as a personal or laptop computer) through USB 2.0, the bus not only provides the data lines for data transfer between the two devices, but +5 volts (nominal) is also provided to the peripheral through the interconnect. Thus, devices coupled onto the USB 2.0 interconnect may utilize the voltage provided on the bus to power circuitry of the peripheral component.
A DC-DC (direct current-to-direct current) converter may be utilized to convert the battery voltage, external voltage or both voltages to a DC value which will supply power to various circuits of an integrated circuit chip or chips. The converted voltage may be a supply voltage (rail voltage) provided to a load. Since the load at the output of the DC-DC converter may vary substantially, the load current may vary considerably as well. For example, with a MP3 player, the load may vary considerably due to the various power states that the MP3 player may be in. When the MP3 player is not being utilized, the MP3 player may have an ultra-low power state, but may awaken as soon as a button is pushed. In some instances it is desirable for the DC-DC converter to be as efficient as possible in the low power state to extend the device's battery life.
However, when switched DC-DC converters are used, the converters are inherently less efficient when driving light loads because the power needed to switch the large power field-effect-transistors (FETs) is comparable or greater than the energy transferred to the load. When driving a heavy load, the energy needed to switch the FETs is less relative to energy transferred to the load, so the efficiency is higher. A common way to improve light load efficiency is to implement a pulse-frequency modulation (PFM) converter, which modulates the frequency that the DC-DC converter operates to optimize efficiency by enabling the power FETs to bring the output voltage to a high threshold, then disabling the FETs until the voltage drops to a low threshold. This improves efficiency by reducing the number of time the FETs are toggled to transfer power to the load.
The architecture of these PFM converters typically implement some form of current mode control. However, current mode control is difficult, because it requires sensing currents, which is difficult to sense due to noise, offsets and circuit complexity. Also, sensing a current usually requires a sensing resistor, which negatively impacts efficiency. Thus, sensing current in a switched DC-DC converter may be difficult to do or may be undesirable.